1. Field of the Invention
This invention relates generally to control of a variable displacement hydraulic pump and more particularly to control of an eccentric rotor vane pump.
2. Description of the Prior Art
A variable displacement hydraulic pump provides variable flow rate output depending on the transmission system flow requirement, also known as “flow demand”. Change of the flow output is achieved by changing displacement of the pump or volume of fluid transported from inlet to outlet per one revolution of the pump's input shaft. Flow output could be maintained constant when pump rotational speed increases.
Variable displacement hydraulic pumps are used for many automatic transmissions. Unlike a variable displacement hydraulic pump, another type of transmission pump, a gear pump, uses a fixed displacement that provides hydraulic flow directly proportional to pump's rotational speed.
The power hydraulics and control system for an automatic transmission require that flow rate supply (flow demand) be proportional to pressure required at a given temperature and not affected by pump speed.
System hydraulic pressure is regulated by an electronically controlled hydraulic valve and maintained by a hydraulic control system that exhausts an excess of hydraulic flow to transmission sump.
A difference between the hydraulic flow required and the delivered flow at pressure and temperature results in excessive hydraulic flow at pressure, thereby producing hydraulic losses that are proportional to pump's hydraulic output, which is proportional to pump speed.